1. Field of the Invention
This invention relates to a process by which improved operation of a continuous coal deashing process can be effected, and more particularly but not by way of limitation, to a process for maintaining the fluidity of fluid-like phases formed during the deashing process.
2. Description of the Prior Art
Various coal deashing processes have been developed in the past wherein coal has been treated with one or more solvents and processed to separate the resulting insoluble coal products from the soluble coal products.
U.S. Pat. Nos. 3,607,716 and 3,607,717 assigned to the same assignee as the present invention describe processes wherein coal liquefaction products are contacted with a solvent and the resulting mixture is introduced into a separation zone maintained at elevated temperature and pressure and separated into a heavy phase containing the insoluble coal products and a light phase containing the soluble coal products. In such processes, the light phase is withdrawn from the separation zone and passed to downstream fractionating vessels wherein the soluble coal products are separated into multiple fractions. One means of removing the heavy phase from the elevated temperature and pressure separation zone is to withdraw the material through a conduit in which a pressure reduction valve is positioned.
The separation is effected in these processes by maintaining rigorous control of the process conditions. The failure to maintain the required conditions often will cause the process to become inoperable.
In the event the process conditions are not maintained, the heavy phase, which exhibits fluid-like properties under the operating conditions of the process, begins to solidify into a solid mass upon the interior surfaces of the apparatus comprising the separation zone. Once formed, the solidified mass will not regain the fluid-like properties formerly exhibited by the heavy phase upon a return of the process conditions to their former limits. The solidified mass, when formed in small amounts, exhibits a tendency to spall away from the surfaces of the separation apparatus and settle within the heavy phase within the apparatus. When the heavy phase is withdrawn from the separation zone, the solid fragments therein can cause the pressure reduction valve interposed in the withdrawal conduit to plug or other downstream apparatus can be plugged. If plugging occurs, process operation must be terminated until the plugged apparatus can be removed, cleaned and reinstalled.
In practice, it has been found that if the process apparatus is depressurized and cooled to permit cleaning of plugged apparatus while the heavy phase is permitted to remain within the apparatus, it is not possible to return the heavy phase to its previous fluid-like condition by reheating and repressurizing the process apparatus.
Further, when attempts have been made to isolate, for example, a plugged pressure reduction valve with block valves to permit cleaning of the plugged valve without depressurization of the process apparatus, it is not possible to return to normal operation merely by reopening the closed block vlaves. It is found that upon reopening the valves, solvent flashes from the heavy phase and the heavy phase becomes a solid mass which replugs the pressure reduction valve. In addition, injection of an inert gas, such as nitrogen, at system pressure into the conduit between the block valve and the pressure reduction valve, before reopening the block valve, is not successful in preventing immediate replugging of the pressure reduction valve upon an attempt to resume normal operation.
It would be desirable to provide a means by which the previously described coal deashing processes can be maintained at operating conditions and the heavy phase can be retained in its fluid-like condition while problems causing interruption of the flow of withdrawn heavy phase are corrected.